1. Field of the Invention
The present invention relates to a method for inspecting height and a height inspection apparatus to carry out that method, especially a method and a height inspection device to carry out that method to inspect efficiently and with good precision the height of bumps formed on the surface of a semiconductor chip.
2. Description of the Related Art
A plurality of bump terminals, as input/output terminals, is arranged in a matrix on the surface of a semiconductor chip and the semiconductor chip is placed face down and bonded to a substrate. Such a semiconductor chip is called a flip chip and is used in multi-chip modules, etc.
These bump terminals are spherical terminals formed on electrodes formed on the surface of a semiconductor substrate; they include metal bumps formed with a plating method and solder bumps formed with a solder dip method.
A chip on which a plurality of bump terminals is formed is bonded face down on a substrate. In that case, each bump terminal must connect properly with the corresponding electrode on the substrate. Consequently, it is necessary that the form and height of the bumps be uniform. A height less than other bumps causes the connection with the electrode on the substrate to be no good. When the height is greater and the bump form large, the risk is of a short circuit with adjacent bumps in the bonding step.
Therefore, generally the height of bumps on the surface is inspected before a flip chip is bonded. General methods for inspecting height include the triangulation method wherein light is irradiated, the reflected light thereof is imaged on a split photodiode and PSD (Position Sensitive Detector), and the height detected. Consequently, the surface is scanned with laser beams and the height of each bump, according to the position of the reflected light, is inspected.
However, in the case of scanning spherical bumps with laser beams, the reflected light thereof causes a very irregular reflection in the vicinity of the bump and has a very high reflectance at the apex of the bump, while the reflectance on the surface outside the bump zone is comparatively constant. Because the reflectance of the surface varies greatly in this way, the intensity of the reflected light varies greatly when the intensity of the radiated light is constant. This is the reason why the quantity of reflected light deviates from the dynamic range of the PSD and photodiode; and the problem is that the height cannot be correctly measured.
Also, the method of measuring height directly from the position of the reflected light imaged on the position sensitive detector (PSD) with the triangulation method discussed above is effective for measuring the height of multiple bumps on a semiconductor chip surface. However, the direction of the reflected light is not constant according to the reflection position of the incident light on the bumps due to the curved shape of the bumps. As a result, errors occur in the height detected with the position sensitive element. Also, the focal displacement of the optical system also results in errors in the height detected. Moreover, the direction of those errors differs according to whether this was an anterior focal point or a posterior focal point. Furthermore, the position of the apex of the bump detected is displaced due to the displacement of the optical axis of the optical system to receive the reflected light; the detected height includes some error.
Furthermore, another problem is that an acousto-optic deflector is used for high speed scanning with laser beams of an LSI (large scale integration) chip having multiple bumps. The frequency variation signal supplied to the acousto-optic deflector is produced with a voltage oscillation circuit, for example. However, the problems include the problem of the astigmatism of this acousto-optic deflector, the problem of a dynamic variation of the astigmatism according to the distorted output characteristics of the voltage controlled oscillator, and the problem of the tolerance of the optical system. Because of these problems, it is very difficult to effect scanning of the measured object at a uniform speed and with zero astigmatism.
These problems are all problems brought about in the case of detecting the height of multiple bumps on a LSI chip. The resolution of all these problems is necessary for accurate height inspection.